1. Field of the Invention
The present invention relates to a method and a control circuit for controlling a drive of a phase splitting start single-phase induction motor. More particularly, the present invention relates to a method and a control circuit for controlling a drive of a phase splitting start single-phase induction motor used in a shredder.
2. Description of Related Art
Sometimes, there is a shredder that uses a phase splitting start single-phase induction motor having a mechanical governor switch, as a driving source for a cutting mechanism. In such a shredder, at a time of a start, a current flows into a main wiring of the motor, and a current flows through a mechanical governor switch, which is on, even into an auxiliary wiring. Then, when a rotation speed of the motor is increased to a predetermined speed, the mechanical governor switch is turned off. As this result, since the current of the auxiliary wiring is shut off, the motor is then rotated only by the current flowing into the main wiring, and the cutting mechanism is driven.
FIG. 1A and FIG. 1B are timing charts showing the rotation speed of the motor when the conventional phase splitting start single-phase induction motor having the mechanical governor switch is used in the shredder (FIG. 1A) and the on/off state of the mechanical governor switch (FIG. 1B).
In the above mentioned conventional shredder, when a paper whose quantity exceeds the performance of the cutting mechanism is supplied while the paper is cut, the paper cannot be cut. In this case, as shown in FIG. 1A, the rotation speed of the motor is decreased. Then, when the rotation speed of the motor is decreased to a predetermined switching speed (at a point C) to be fed the current to the auxiliary wiring, the mechanical governor switch is again turned on, and the current again flows into the auxiliary wiring. Thus, the rotation speed of the motor is increased, and an output torque is also made larger (shown by a curve D between the point C and a point A), which enables the paper to be again cut.
Then, when the rotation speed of the motor is gradually increased and reaches the above mentioned switching speed (at the point A), the mechanical governor switch is again turned off. In this way, when the mechanical governor switch is again turned off, the output torque of the motor begins to be short (shown by a curve B between the point A and the point C). As this result, the paper again cannot be cut. In this case, the rotation speed of the motor is again decreased to the switching speed (at the point C). Hence, the mechanical governor switch is again turned on.
The conventional shredder has a problem of an occurrence of a chattering phenomenon where as mentioned above, the mechanical governor switch repeats the on/off state, as shown in FIG. 1B.
Also, when the chattering phenomenon is generated in the mechanical governor switch, the output torque of the motor is rippled. As this result, impact force is repeatedly applied to a cutter of the shredder. Thus, there is a problem that the cutter is deformed and a blade is spilled.
Also, in the above-mentioned situation, the mechanical governor switch repeatedly turns on and off the large load current flowing into the motor in the overload state. Thus, when contacts of the mechanical governor switch are opened (it is turned off), electric spark is generated between the contacts. This fact brings about a problem of parts life that the contact life of the mechanical governor switch is made shorter. Moreover, inside the shredder, there is a safety problem that the generation of paper dust causes the possibility of fire.
Also, there is a problem that the outer shape of the phase splitting start single-phase induction motor becomes larger when the mechanical governor switch is installed.
It is noted that the mechanical governor switch can be adjusted so as to determine the timing when the current flowing into the auxiliary wiring is shut off, in accordance with the rotation speed of the motor. However, in the case that the current again flows into the auxiliary wiring at the time of the overload, it is very difficult to adjust to the proper rotation speed so that the contacts of the mechanical governor switch are not chattered.
In view of the above-mentioned problems, instead of the mechanical governor switch, the use of a switching circuit of a non-contact type has been variously proposed as described below.
Japanese Patent Application Laid-Open No.59-28889 (1984) discloses a start control apparatus of a single-phase induction motor for cutting off an auxiliary wiring when a voltage induced in a main wiring becomes higher than that in an auxiliary wiring.
Japanese Patent Application Laid-Open No.2000-32786 discloses a starter for a single-phase induction motor for cutting off an auxiliary wiring based on an electromotive force induced in an auxiliary wiring.
Japanese Patent No. 2948902 discloses an electronic circuit for starting a single-phase induction motor which cuts off an auxiliary wiring at a predetermined time after a start.
Japanese Patent Application Laid-Open No. 2005-151625 discloses a governor switch for cutting off an auxiliary wiring based on a motor rotation speed.
Japanese Patent Application Laid-Open No. 64-43081 (1989) discloses a motor start control circuit which carries out an acceleration or restart, by feeding a current into an auxiliary wiring, in a case of an overload state.
However, the above-mentioned problem that the contacts of the mechanical governor switch are chattered cannot be said to be solved by any of the conventional techniques.